A cathode-drive laser driver can be represented in a simplified matter as a current source which can drive a current through a laser. FIG. 1 shows schematically such a back terminated laser driver. High-speed cathode-drive laser drivers are typically implemented with a back termination resistor to move the dominant pole to higher frequencies.
For back-terminated cathode-drive laser drivers the anode voltage of the laser affects the average laser current and needs to be set at a specific value in order to minimize this impact.
The back termination resistor Rt (resistor 11 in FIG. 1) not only reduces the drive efficiency (m) to the laser, but also introduces a DC current path between the supply voltage node of the driver Vdd1 (the voltage at node 121 in FIG. 1) and the anode voltage node of the laser 195 Vdd2 (the voltage at node 191 in FIG. 1).
In the ideal situation the current through the laser only depends on the current from the driver but as there are two supply voltages there is an extra current path between the supply voltage node of the driver Vdd1 and the anode voltage node of the laser Vdd2. This extra current depends on the difference between the supply voltages Vdd1 and Vdd2. The current can be a negative or a positive current, of which the value is hard to predict or control because of the dependence on Rt and on the series resistance of the laser R1 (the laser can be modelled as a voltage source Vlth and a resistor RI at DC), hence a constraint on the anode voltage arises that minimizes the extra undesired current through the laser.
In typical driver designs Vdd2 is higher than Vdd1 with an amount equal to the threshold voltage drop of the laser (Vlth in FIG. 1). The laser supply voltage is therefore typically 1 to 2 Volt higher. Therefore it is difficult to achieve very low power consumption and this also complicates the external power supply circuitry to power the chip or transceiver module. Laser technology may for example be operated at 3.3 V while transistor technology may typically be operated at 1-2 V.
The graph in FIG. 1 shows the optical power as function of the laser current Il. The graph shows the relation between the modulation current Im, the bias current Ib and the average laser current Īl.
The relationship between the laser current Il and the driver current Id is as follows:
      I    l    =                    I        d            ·      m        +                            V                      dd            ⁢                                                  ⁢            2                          -                  V                      dd            ⁢                                                  ⁢            1                          -                  V          lth                                      R                      t            +                          ⁢                  R          l                    wherein m is the drive efficiency and can be calculated as follows:
  m  =                    R        t                              R          t                +                  R          l                      <    1  and wherein the second term in the equation is the extra current through the laser. When targeting energy efficient ICs for future data center interconnects, it is advantageous that the anode voltage Vdd2 can be lowered as much as possible, thereby pursuing single-supply operation for the laser driver.